Infrared radiation sources, sensors and source combinations, and methods of manufacture

ABSTRACT

A blackbody radiation device ( 110 ) includes a planar filament emission element ( 102 ) and a planar detector ( 104 ) for respectively producing and detecting radiation having width dl/1 less than about 0.1 to test a sample gas, where 1 is the wavelength of the radiation; a reflector ( 108 ); a window (W); an electrical control ( 118 ); and a data output element ( 116 ).

RELATED APPLICATIONS

This application is a continuation-in-part of commonly owned and U.S.patent application Ser. No. 08/905,599, filed on Aug. 4, 1997, now U.S.Pat. No. 5,838,016, granted Nov. 17, 1998, and U.S. Provisional PatentApplication Ser. No. 60/094,602, filed on Jul. 30, 1998, and PCTApplication No. PCT/US98/25771, filed Dec. 4, 1998, InternationalPublication No. WO99/28729, which are all hereby expressly incorporatedby reference. Related PCT Application No. US99/07781, filed Apr. 9,1999, is an appendix to this application.

FIELD OF THE INVENTION

This invention relates to optical radiation sources and relates inparticular to blackbody radiation sources whose wavelength-dependentsurface emissivity is modified in order to radiate with higher powerefficiency and/or to radiate within a desired wavelength controlledspectral band. The invention also relates to sensors incorporatingsources and to systems and methods embodying same.

BACKGROUND OF THE INVENTION

Non-dispersive Infrared (NDIR) techniques utilizing the characteristicabsorption bands of gases in the infrared have long been considered asone of the best methods for gas measurement. These techniques takeadvantage of the fact that various gases exhibit substantial absorptionat specific wavelengths in the infrared radiation spectrum. The term“non-dispersive” refers to the type of apparatus incorporating the NDIRtechnique, typically including a narrow band pass interference filter(as opposed to a “dispersive” element such as a prism or a diffractiongrating to isolate and pass radiation in a particular wavelength bandfrom a spectrally broad band infrared source. The gas concentration isdiscerned from the detected intensity modulation of source radiationemanating from the source and passed by the filter coincident inwavelength with a strong absorption band of the gas to be measured.

A prior art NDIR gas analyzer typically includes an infrared source witha motor-driven mechanical chopper to modulate the source so thatsynchronous detection is used to discriminate spurious infraredradiation from surroundings; a pump to push gas through a samplechamber; a narrow band-pass interference filter; a sensitive infrareddetector, inexpensive infrared optics and windows to focus the infraredenergy from the source onto the detector. Despite the fact that the NDIRgas measurement technique is one of the best methodologies that had everbeen devised, it has not enjoyed wide application because of itscomplexity and high cost of implementation.

Several components play essential roles in making the NDIR techniquework for gas measurement. The radiation source is one such component;and in order for NDIR methodology to work efficaciously, this sourcemust provide broad spectral output and give high power in the band ofinterest. A blackbody radiation source is generally used for thiscomponent because it can be heated to a temperature that provides highintensity radiation within any wavelength region. Because the blackbodyoutput spectral distribution and intensity are uniquely determined bythe temperature of the source, the spectral peak intensity can beadjusted by varying the temperature of the blackbody source. By thewell-known Wien's Displacement Law the peak intensity of a blackbodyradiation source of temperature T is at a wavelength λ_(max) equivalentto 2.898×10−³/T where T is measured in degrees Kelvin (1° K) and λ_(max)is measured in meters. By the process of T and the size of the blackbodyradiator, a desired intensity of blackbody radiation selected wavelengthλ can be attained.

The so-called “Nernst Glower,” consisting of a heating element of atungsten filament) embedded in a ceramic slab, has long been used forthe blackbody source of most prior art NDIR gas detection systems.Despite the fact that the Nernst Glower gives off an amount of infraredenergy with an emissivity close to unity, its power efficiency (i.e.,efficiency from electrical to useable optical energy) is notoriouslylow. The large amount of unwanted heat is also a major drawback in theuse of the Nernst Glower in any NDIR gas measurement systems.Furthermore, the large heat capacity of the Nernst Glower makes it anecessarily slow device in terms of intensity modulation. In many cases,it can only be used as a steady state or DC radiation source; and thus amechanical chopper is needed to generate synchronous modulated signals,further adding to the complexity of NDIR gas measurement systems.

The prior art has attempted to replace the blackbody radiator with hotfilament as an approximation to a blackbody. However, such a filamentdoes not have high emission over all wavelengths because there isspatial temperature variation within the filament; it is not an idealblackbody source. Optical radiation sources, such as a hot filamentlamp, are thus often referred to as “quasi-blackbody” sources. Inaddition, hot filament radiators typically are used by a quartz bulbwhich is substantially opaque for wavelengths, longer than about 4.5 μmreducing its applicability to gas detection through longer wavelengthabsorption lines. Prior art hot filaments are thus not particularly goodinfrared radiation sources for use with NDIR gas sensors.

The prior art has attempted to improve NDIR sensors. In U.S. Pat. No.4,876,413 (the '414 patent), published in April of 1975, Bridghamdescribes an infrared radiation source that includes a thin filmresistor heater with highly emissive material Cr₃Si on a substrate. Thethin film heater is positioned between a pair of thin metal elementsserving as sensing electrodes on a very thin (<0.005″ typical)insulating substrate. The entire thin film heater is packaged in astandard TO-5 heater equipped with a focusing reflector and pinssupporting the heater structure.

While the source of the '413 patent advances infrared radiator sourcesin terms of higher emissivity and wider spectral emissions, it does notoffer size or speed advance over the classic tungsten lamp. Furthermore,its construction is rather fragile and the heater cannot withstandtemperatures above −700° C., severely limiting the maximum allowableoutput. Finally, its overall power efficiency is rather poor and the lowcost and useable life of this radiator has not been satisfactorilyproven. Consequently, the source of the '413 patent has not found wideapplication in NDIR gas measurement systems.

In U.S. Pat. No. 4,644,141 published in February of 1987, Hager et al.advances a heater structure first proposed in the '413 patent, exceptthat a combination of silicon, silicon oxides and a platinum metalpattern are used as the heating element to optimize the performance ofthe overall heater structure. Nevertheless, other than a slightimprovement in power efficiency over the device of '413 patent, there isno fundamental advancement over the prior art.

Over the past several years, significant technical progress has beenmade in the area of optical sources as set forth in the inventor's ownprior application of U.S. Ser. No. 08/511,070, filed on Aug. 3, 1995.Such optical sources have greatly increased the reliability andcost-effectiveness of NDIR gas sensors.

The present invention has several objects in providing furtherimprovements and advantages in source and sensor embodiments as comparedto the prior art. One object of the invention is to provide sourceswhich function as tuned waveband emitters that preferentially emitradiation into a wavelength band of interest as compared to blackbody orgray-body radiators. Another object of the invention provides inintegrated circuit sensor include selectively tuned radiation source.Still another object of the invention provides methods and devices forsensing gas constituents without certain of the difficulties andproblems of prior art NDIR devices.

These and other objects will become apparent in the description whichfollows.

SUMMARY OF THE INVENTION

As used herein, “on chip” refers to integrated circuits and the likewhich are processed through microelectronic fabrication techniques toprovide circuitry and/or processes (including A/D processing) on asemiconductor element or chip.

As used herein, “SOURCE” generally means tuned waveband emittersconstructed according to the invention that preferentially emitradiation into a wavelength band of interest. Certain SOURCES, forexample, include those tuned radiation sources described in the commonlyassigned and copending U.S. Ser. No. 08/905,599, filed on Aug. 4, 1997,now U.S. Pat. No. 5,838,016, granted Nov. 17, 1998, and U.S. Ser. No.08/511,070, filed on Aug. 3, 1995, now abandoned (the latter also beingincorporated herein by reference). Still other SOURCES including tunedcavity emitters, chemical and microelectronic/semiconductor emitters,and other etched electro-mechanical, chemically-treated, ion-bombardedand lithographically created surfaces which permit spectral control andwhich provide narrow band incoherent emissions tailored to specificend-user needs and requirements.

The invention has many objectives in providing improved sources andsensors. It also utilizes and is complimentary to certain artencompassing other areas including NDIR gas measurement technology,silicon micro-machined thermopile, optical beam configurations andfabrication techniques. As such, the following patents and publicationsproviding information and background to the specification and are hereinincorporated by reference. U.S. Pat. No. 4,620,104; U.S. Pat. No.4,644,141; U.S. Pat. No. 4,859,858; U.S. Pat. No. 4,926,992; U.S. Pat.No. 5,060,508; U.S. Pat. No. 5,074,490; U.S. Pat. No. 5,128,514; U.S.Pat. No. 5,152,870; U.S. Pat. No. 5,220,173; U.S. Pat. No. 5,324,951;U.S. Pat. No. 5,444,249,249; Hutley et al., The Total Absorption OfLight By A Diffractive Grating, Optics Communications, 19(3), pp.431–436 (1976); Loewen et al., Dielectric Coated Gratings: A CuriousProperty, Applied Optics, Vol. 16(11), pp. 3009–3011 (1977); Rajic etal., Design, Fabrication, And Testing Of Micro-Optical SensorsContaining Multiple Aspheres, SPIE, Vol. 2356, pp. 452–462 (1995); Guptaet al., Infrared Filters Using Metallica Photonic Band Gap Structures OnFlexible Substrates, Appi Phys. Lett., 71(17), pp. 2412–2444 (1997); andRehse et al., Nanolithograhy With Metastable Iron Atoms: Enhanced RateOf Contamination Resist Formation For Nanostructure Fabrication, Appl.Phys. Lett., 71(10), pp. 1427–1429 (1997).

In one aspect, the invention provides an integrated circuit sensor(ICS). The ICS has a tuned narrow band emitter (e.g., a SOURCE of theinvention) and a tuned narrow band detector fabricated on a singlesilicon die, Optics (e.g., reflective optics or a silicon slabwaveguide) capture source emissions and refocus the captured energy ontothe detector. Control electronics are preferably included and arepreferably incorporated “on chip” to facilitate packaging issues. Asolar cell can also be integrated with the sensor to provide power. Thesensor can further be integrated with other features described herein orcontained in related applications.

In another aspect, the invention provides a Hybrid Infrared Gas andThermal Sensor Head (“HIRGS”). A SOURCE is integrated with an uncooleddetector (e.g., a microbolometer detector), and a reflector (e.g., aparabolic reflector) into a package such as a single transistor can.Source drive electronics and detector readout electronics and preferablyincluded with the HIRGS and can also be provided “on chip”. The detectoris also preferably “tuned” to the waveband emissions of the SOURCE.

In still another aspect, the invention provides an Integrated Gas Sensor(“IGS”). A SOURCE is coupled to illumination optics which pass a beam(of tuned magnetic radiation) into a gas sample or stream. A detector(preferably with receiving optics) is arranged to capture the beam andconvert the beam into electrical signals. An electronics system is usedto diagnose the beam intensity and spectral characteristics to analyzethe gas, e.g., to determine the % concentration of a particular gas.

In another aspect, an alternative IGS includes a SOURCE whichincorporates a gas cell sample (or gas flow cell). Wavelength compatibleoptics preferably collect SOURCE emissions to efficiently illuminate thecell. A mirror (e.g., a flat mirror) positioned on the opposite side ofthe cell reflects the SOURCE emissions back through the cell and to adetector (also preferably with optics to improve collection efficiency).The detector is positioned to collect the reflected beam adjacent to theSOURCE. An electronics subsystem is used to diagnose the beam intensityand spectral intensities to determine characteristics of the gas, e.g.,concentration of a particular gas.

In one aspect, methodology is provided for manufacturing amicrobolometer-based SOURCE and integrated sensor. The sensor includesone microbolometer arranged to emit tuned radiation, optics to collectand transmit the radiation to a sample under study for a specificpurpose (e.g., to illuminate a gas cell or a solid surface), and anothermicrobolometer arranged to collect at least part of the post sample(i.e., transmitted through the gas or scattered from the surface)radiation. As known in the art, microbolometers function by efficientlycollecting infrared radiation within an absorbing microbridge cavity;and the absorbed energy changes the resistance of the bridgeelectronics, indicating an amount of radiation. The efficient thermalmass of the bridge permits very fast dissipation of radiant energy sothat multiple microbolometer can efficiently take high speed IR pictureframes (i.e, each microbolometer operating as a pixel in a picture inaccord with the invention, the source microbolometer operates inreverse: electric energy drives the microbolometer to radiate heat andto emit radiation in the band of the cavity. The sister microbolometerdetector is tuned to the same band and the two microbolometereffectively emit and recover tuned electromagnetic radiation.

In another aspect, a tuned cavity band emitter SOURCE is provided. TheSOURCE of this aspect has an emissivity curve which differs—selectivelyand beneficially-from a standard blackbody curve. It thus functions as aselective incoherent band emitting infrared source. This SOURCE can thusinclude a metal foil filament such as provided in U.S. patentapplication Ser. No. 08/905,599; and can further include a metaltransistor can be coupled to the SOURCE. The SOURCE of this aspect canalso be constructed with spectral control, including supportingelectronics, to function as an incoherent narrow band emitter.Preferably, the surface of the emitting SOURCE of this aspect is dark.It also includes a sharp spectral transition between a high absorbingband and a highly reflecting out of band, thereby functioning as a “bandemitter”.

By way of background, prior art incoherent sources radiate in a mannerrepresentative of a blackbody curve. However, most applications andproducts utilize only a small band within the spectrum relative to thatblackbody curve (by way of example, most devices are used to generateenergy within a defined band; and other radiation can result in noise).These prior art sources thus inefficiently emit energy into the band.Accordingly, when the prior art source is brought to temperature, theblackbody emissions at the center of the measurement band represent onlya tiny fraction of the total radiant output falls within the desiredmeasurement band. The invention solves this dilemma.

In accord with the invention, the SOURCE emitter surface of one aspectis modified to become an effective band emitter. By suppressingout-of-band emission sensors and systems utilizing the SOURCE tradeout-of-band photons (noise) for in-band photon (signal). In essence, theSOURCE becomes a near-perfect reflecting surface for out-of-bandradiation and a highly efficient absorber for in-band radiationcorresponding to an array of absorbers. The size, shape, and spacing ofthese cavities are designed to absorb at a pre-selected resonancewavelength. The size of these cavities should be about λ/2π for thedesired wavelength of medium and longwave infrared radiation (e.g., 3–5μm and/or 8–12 μm), the cavity sizes are within the limits of state ofthe art microlithography and microfabrication techniques. Thus, thisaspect of the invention includes a SOURCE with manually applied cavitiesdisposed thereover with microlithography.

In another aspect, the invention provides mid-and long-wavelengthinfrared sensors for gas detector applications utilizing anon-blackbody, narrow-band radiator. The SOURCE of this aspect includessub-micron scale surface structures which facilitate control of theradiator emission spectrum. The source thus combines blackbody radiatorfunctionality with a surface that selectively filters wavelengthemissions.

In another aspect, an optical radiation SOURCE is provided whichoptimizes the following three parameters for improved wavebandemissions: operating temperature T(° K), emitting area A, and surfaceemissivity ε(λ). The SOURCE utilizes structure which emphasizes theemitting surface characteristics (e.g., cavity structure) of theradiator and which optimizes its emitting area and its emissivity tomaximize power output in the optimized waveband.

Another aspect of the invention provides an optical radiation SOURCEwith an output that is “spectrally tailored” relative to the emission aperfect blackbody by modifying surface emissivity so as to maximizepower output in a desired and controlled spectral wavelength.

In another aspect, the optical radiation SOURCE of the invention has athin structure with a well-defined outwardly facing surface that emitsoptical radiation. The thin structure includes a resistive element inthe form of a thin film or foil supported by a substrate and configuredwith surface structure as discussed above. The optical radiator SOURCEis powered by electrical current passed through its resistive element.The texture of the outwardly facing filament is methodically andcontrollably modified by well-known techniques—such as electrolyticplasma etching, ion-milling, and preferably in combination withphoto-lithographic masking technology—such that the emissivity of theradiator SOURCE possesses an emissivity near to unity with well-definedand controllable spectral wavelength band.

The invention is next described further in connection with preferredembodiments, and it will become apparent that various additions,subtractions, and modifications may be made by those skilled in the artwithout departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of this invention, the various featuresthereof, as well as the invention itself, may be more fully understoodfrom the following description, when read together with the accompanyingdrawings in which:

FIG. 1A shows an IR source in accordance with the invention;

FIG. 1B shows the beam shape of the IR source of FIG. 1;

FIG. 1C shows a graphical comparison of drive power of the IR source ofFIG. 1 and a standard source;

FIGS. 1D, 2 and 3A show alternative IR sources in accordance with theinvention;

FIGS. 3B, 3C and 3D show exemplary filament emitters in accordance withthe invention;

FIG. 4 shows another IR source in accordance with the invention;

FIGS. 5 and 6 show additional exemplary emitters;

FIG. 7 shows the spectral irradiance as a function of wavelength for atextured metal foil filament of the invention;

FIGS. 7A(a) and 7A(b) show exemplary mask patterns for an IR sourceaccording to the invention;

FIGS. 7A(c), 7B and 7C show the emission spectra of IR sources of theinvention;

FIGS. 7D(a) and 7D(b) show emission wavelength versus etched cavity sizeand versus cavity-to cavity spacing, respectively;

FIG. 8A of sheet 8/17 of WO 00/07411, and FIG. 8B(a)–8B(c) illustrateexemplary window frame construction steps for forming individualradiator elements on a silicon die in accordance with the invention;

FIGS. 8A and 8B of sheet 4/17 of WO 00/07411 are duplicative of step 7and step 10 of FIG. 8A of sheet 8/17 of WO 00/07411;

FIGS. 9A and 9B show conversion efficiency gains using lithographicfeature design and fabrication according to the invention;

FIG. 10 shows an exemplary IR hydrocarbon leak sensor in accordance withthe invention;

FIGS. 11A and 11B show an exemplary IR gas sensor engine in accordancewith the invention;

FIGS. 11C–11I show other exemplary devices using separate radiationsources and detectors in accordance with the invention;

FIG. 13 shows in schematic form, a test configuration for the invention;

FIG. 14A shows an exemplary nondispersive test bed using the invention;

FIG. 14B shows in block diagram form, an exemplary signal processing fordate reduction for the test bed of FIG. 14A;

FIG. 15 shows a calibration spectrum for a hyperspectral imaging systemof the invention;

FIG. 16 is duplicative of FIG. 9A;

FIGS. 17A and 17B show an exemplary resolution test pattern and aresolution tester respectively for the invention;

FIGS. 18 and 18B show an exemplary system of the invention;

FIG. 19A shows an exemplary reference source according to the inventionin a filter wheel slot; and

FIG. 19B shows exemplary signal sources adjacent to a metric.

DESCRIPTION OF PREFERRED EMBODIMENTS

I. Reflector Source with Reflector Housing in Front of Planar Emitterwhich is Normal to the Emission Axis, in Conjunction with TexturedSource

One form of the invention is a relatively low power reflector infrared(IR) source with an improved texture, smaller (compared to the priorart) filament, and with an integral reflector.

A common design issue for IR sources is the requirement forsignificantly more useful signal, particularly in the LWIR, withsignificantly less required drive power. The present invention effectssuch changes by including reduced filament size, improved filamenttexture, and by incorporating a reflector into the source package. Thisallows, for example, for medical instruments (including anesthesiamonitors or critical care systems), industrial safety instruments, andautomotive emission monitoring.

FIG. 1A shows an IR source 10 of the invention having relatively highusable signal with relatively low drive power. A relatively smallfilament 10 is used with a molded reflector 12 to provide a “searchlight” type beam-former, illuminating a narrow forward cone (as shown inFIG. 1B) and eliminating the need for discrete optical (lens) elements.The smaller filament of the invention also uses less power compared tothe prior art (“standard”) as shown in FIG. 1C.

This source 10 reduces the required parts count and integrationcomplexity for instrument environments, by integrating relatively highfunctionality into a single component by reducing or eliminating theneed for separate optical elements and providing an output illuminationcone which is compatible with standard thin film interference filters.

FIG. 1D shows an IR source 10′ which is similar to source 10 butincludes an “M”-shaped filament 11′, and a formed-sheet reflector 12′.

This form of IR source does not require extensive tooling or automation.Preferably, the source utilizes a laser cut filament cut from thinself-supporting metal foils which are treated in large (e.g., 10 cmsquare) sheets. Again, preferably, the filament size is relatively small(for example, 250 parts per 10 cm square).

II. Radiation Source with Parabolic or Compound Parabolic “Winston”Collimator, in Conjunction with Textured Source

FIG. 2 shows another exemplary form of IR source of the invention,particularly including straight parabolic, or compound paraboliccollimators configured in combination with a textured source. Theembodiment of FIG. 2 shows a single loop, single bar filament with a“behind the source” concentrators. FIG. 3A shows a “two-sided” emitterwith a “behind the source” concentrator. FIGS. 3B–3D show exemplaryfilament emitters.

FIG. 4 shows a “flat pack” construction source which allows a variety ofreflector configurations. By utilizing double sided emission, thefilament effectively doubles its active area and provides twice theusable in-band signal flux at a given temperature.

Other filament configurations provide additional “shine-through”illumination for rear-mounted reflectors, for example, as shown in FIGS.5 and 6. In FIG. 6, the filament is “sideways”, i.e., the reflector axisis in the plane of the filament.

III. Tuned Band Emitter Sources

A. Tuned Band Emitter “Photonic Band Gap” Filament

The incorporated PCT Application No. PCT/US98/25771 discloses IR sourceswith metal foil filaments, and particularly shows an embodiment in a Txmetal transistor can. That form of source effects spectral control, andteaches use of spectral control to construct an incoherent narrow bandemitter. In the prior art, the emitting surface has been modified tomake it dark, but significant spectral control has not beenaccomplished. In accordance with the present invention, there are sharpspectral transitions between highly absorbing, in band, and highlyreflecting, out of band, thereby effecting a band emitter. Traditionalincoherent sources are constrained by the blackbody curve. The blackbodycurve assures that, even if the source temperature is brought to peakwith the blackbody distribution at the center of the measurement band,only a small fraction of the total radiant output falls within thedesired measurement band.

In accordance with the present invention, the emitter surface (and theconsequent surface spectrum) is shaped and textured so that theblackbody curve helps to define a band emitter. By suppressingout-of-band emission, this allows an instrument designer to tradeout-of-band photons (noise) for in-band photons (signal.) This iseffected with a highly reflecting surface having an array of absorbingcavities thereon. The size, shape, and spacing of these cavities areestablished to absorb at a pre-selected resonance wavelength. Asdetermined from incoherent scattering theory, the size of these cavitiesis preferably about 1/2p for the desired wavelength. For wavelengths inthe MWIR and/or LWIR gas bands, these sizes are well within the limitsof current microlithography and microfabrication techniques.

The blackbody curve naturally rolls off more steeply on the shortwavelength side, so it naturally defines the short wavelength band edge,even if the surface emissivity does not exhibit a sharp short wavelengthcut-off.

Spectral Control

The small surface texture features which make the infrared source of theinvention work are preferably made by a random seed texturing process.These features make the surface a selective emitter with fullblackbody-like emissivity at short wavelengths but little or noemissivity at longer wavelengths.

Spectral selection is determined using random textured infrared emittersfor gas detector applications. A non-blackbody, narrow-band radiatorreplaces the function of a traditional interference filter anddramatically improves the efficiency of the surface as an in-bandemitter. By building a surface which is dark (high emissivity) in-band,and shiny (low emissivity) outside that band, the ratio of usablebandpass flux-to-total flux maximized, and therefore maximizinginstrument sensitivity. Sub-micron scale surface structures control theradiator emission spectrum. In effect, the function of the blackbodyradiator surface is combined with the function of the bolometer element,and together with some of the function of a thin film interferencefilter into a single component.

To achieve device miniaturization, it is important to achieve tightspectral control of the infrared emission. This enables maximum infraredabsorption signal with minimum parasitic heating of the neighboringdetector element.

The infrared emitters for gas detection are preferably made using randomseed texturing of the radiator surfaces. FIG. 7 shows spectralirradiance (relative units) as a function of wavelength for a texturedmetal foil filament of the invention, as compared with a conventional575CBB filament. The exemplary metal foil radiator displays spectralnarrowing dl/1˜0.5 (FWHM), by random texture. A mixture of small andlarge surface feature sizes accounts for the spectral width.Lithographic techniques are used to produce well controlled surfacefeatures for narrow emission waveband.

For an ideal blackbody under these conditions, only a few percent of thetotal flux is in-band. But the textured metal radiator surfaces suppresslong wavelength radiation and therefore improves this ratio. Theconversion efficiency of a radiator surface (where neither the surfacepreparation or the operating temperature were optimized for thispurpose) is around 4%. Further improvements can be effected by tuningthe spectral emission band specifically for this purpose and carefullymaintaining the appropriate operating temperature during themeasurement. Preferably, a tuned cavity band emitter with spectralresolution (dl/1) around 0.1, is achieved which is comparable to thatachieved with micromesh reflective filters. This level of surfacetopology (and therefore spectral) control, is achieved usinglithographic surface modification techniques.

An exemplary emission device is based on the mask pattern shown in FIGS.7A(a) and 7A(b). The patterns in the center of the mask labeled 8,10 and12B or N are crosses, either broad (B) or narrow (N) as shown in thedetail. Numbering corresponds to the wavelength (i.e., 8, 10, or 12microns) of the peak absorption. Other patterns can be annular rings andtripoles. The patterns are etched into polished silicon wafers, 1–3, μmdeep. A thin layer of aluminum may be evaporated onto the surface of awafer to change the background emissivity from that of silicon (ε=0.7)to that of aluminum (ε=0.1).

The reflection spectra from the exemplary patterned silicon wafer(without an aluminum coating) are shown in FIG. 8A(c). The absorptionpeaks are consistently shorter than the target wavelengths (by about20%) and depend on the feature size as well as the unit cell size(feature spacing).

In FIG. 7A(c) the spectra are shown as a ratio with the reflectance ofan untreated silicon area on the same wafer. Two areas (12N and 12B,respectively) of the wafer show spectral absorption features related tothe size and spacing of patterns on the wafer surface.

With controllable band emission devices of the invention, thermalemission from photonic bandgap surface structures show distinct peakswith a wavelength proportionally to the geometry of the structure. Thispermits sensor-on-a-chip configurations, for example, where thermalradiation from a “designed” textured surface can be concentrated into anarrow band with low values of Δλ/λ. The sensitivity of the detectorapproaches theoretical limits.

FIG. 7B shows emission spectra from aluminized patterned silicon surfaceheated to 500° C. showing distinct peaks with wavelength varying asgeometrical scale factor. In that figure, all sites on the wafer wereheated to the same temperature. The surface structure causes sharpemission peaks which appear over a spectral range of nearly an octave.The emission peaks on the aluminum coated wafer have shifted to shorterwavelengths. Various embodiments may use coated wafers, uncoated wafers,and wafers with different doping.

FIG. 7B shows a curve fit to the date from FIG. 7B for a cross design asshown in FIG. 7B. The upper (black) curve illustrates the emissionspectrum from an ideal blackbody at 773 K. The lower curve is thesemi-empirical thermal model used for estimating instrument signal tonoise. It consists of a graybody (with emissivity of 0.48) at atemperature of 773 K, plus excess emission at a peak wavelength of 6.89microns and an emission width of Δλ/λ≦0.16. The upper curve in FIG. 7Bshows a measured emission data, demonstrating a short wavelength cutoff.Random ion-beam produced texture provides devices with long wavelengthcutoff.

In FIG. 7C, the background blackbody emission from the silicon wafer hasa relatively high emissivity of at least 0.48 whereas the emissivity ofa good aluminum film is below 0.1. Patterns with a larger fraction ofopen area (and hence cleaner coatings) show sharper emission peaks.

The reflectance and remittance peaks from FIGS. 7B and 7C exhibit asystematic short wavelength-side deviation from their targetwavelengths, but demonstrate that PBG peak position and halfwidth arerelated to size and spacing of the periodic features. FIGS. 7D(a) and7D(b) show emission wavelength versus etched cavity size (measured aswidth of cross' leg) and versus cavity-to-cavity spacing. For a 4.65 μmemitter, the cross' leg is about 1.45 μm wide with center-to-centerspacing of about 4 μm.

IC Compatible Fabrication

Substantially regular patterns on thin foils are formed by plating ordepositing metal films. In one form, a silicon substrate is used as amechanical support for the foil during subsequent processing.

Subtractive processes based on deliberate, lithographic seed islandformation are used with ion beam surface texturing processes.Lithographic masking and exposure schemes are used to achieve therequired patterning. Typical lead solder glass materials, used forfabrication of thick film circuits, have a low sputter removal rate, andthey are screen printed and chemically removed after processing.Diamond-like carbon films, deposited by chemical vapor deposition, haveextremely low removal rates but are not easily patterned. Typical layerthicknesses of the solder glass are a few thousandths of an inch (50–100mm) and this is comparable to the resolution achievable with screenprinting equipment. Conventional microlithography for electronics canproduce lines and spaces at least an order of magnitude smaller (twoorders of magnitude for state-of-the-art VLSI processes) although theability to faithfully transfer a pattern to the metal foil is somewhatlimited by the thickness of the mask layer. Since the typical size ofthe pillars which comprise the surface texture is around a micron,micron scale antenna replication is used.

The following method places a regular array of micron-scale structuresof a refractory metal oxide on the surface of high TCR self-supportingmetal foils. The resultant structure has a surface which is a selectiveinfrared absorber in itself or alternatively, is an improved startingmaterial for texturing using ion milling techniques. The steps listedare conventional photolithographic procedures.

Thin metal foils are deposited on silicon wafers for processcompatibility and ease of downstream handling. Alternatively,self-supporting silicon microbridges are used as the individual emitterelements.

After appropriate cleaning of support disc and foil, the drilled/milledarea of the disc is coated with a film of positive photoresist, forexample, applied with a clean brush, and the foil is placed over theholes and pressed or rolled into good contact with the disc.Effectively, the holes in the disc are covered, the disc is then helddown on a vacuum chuck for the next step.

Positive photoresist is next applied to the entire disc, using aconventional spinner followed by a conventional soft-bake step. The ramprate up to baking temperature is maintained to be relatively low toallow solvent in the resist layer which bonds the foil to the disc to bereleased slowly and thus avoid lifting the foil from the disc. A resistformulation designed for use with reflective substrates is useful forthis step.

Masking, exposure, and development of the top layer of photoresist isperformed in a conventional manner. Clear areas in the mask are positiveimages of the features to appear on the foil. The hard-bake phase drivesoff solvents to make the resist film vacuum-worthy.

In cases where a plasma cleaning step is not possible in the equipmentused for film deposition, the assembly is then given a short exposure toan oxygen plasma in apparatus for ashing photoresist. The objective isinsuring adhesion of the deposited film. Deposition of a refractoryoxide film is next done by laser deposition, which allows maintenance oflow substrate temperatures. The refractory film covers the entiredisc/foil assembly, contacting and, preferably adhering to the foilthrough the openings in the photoresist.

In the next step, the coated disc/foil assembly is soaked in acetone (orphotoresist stripper), in some cases with gentle ultrasonic vibration,to complete the lift-off of photoresist and overlying refractory filmand free the foil from the supporting disc. After separation from thesupport disc, the foil is then cleaned and used “as is”, or mounted inan ion mill for texturing as a self-seeded material.

The most desirable packaging alternative is to leave the metal foilintact on the silicon wafer. However, in some forms of the invention,the radiator elements are heated electrically for ready measurement ofinput power and conversion efficiency. In order to achieve this, theradiator elements are thermally and electrically isolated from theirsurroundings. A “window-frame” pattern of cut-outs in the siliconsubstrate is used so that the actual radiator elements are suspended onthe remaining silicon, as shown in FIGS. 8A and 8B(a)–8B(c).

FIGS. 8A (ten steps, 1–10) and 8B(a)–8B(c) show the basic window-frameconstruction technique for forming individual radiator elements on asilicon die. Elements are left on the silicon wafer as a support duringmicrotexturing and then silicon via's are etched from underneath toproduce suspended radiator elements.

Integrated Source and Drive/Stabilization Electronics

Wavelength Selective Bolometer Surface

Computer modeling is used to determine the optimum surface shape fordesired emission characteristics, based in part on transmission throughconductive screen and dielectric-mounted mask arrays. In such models, anincoming electromagnetic wave is considered to produce surfaceexcitations (plasmons) in the metallic screen or grid, and this in turnproduces the transmitted wave. The plasmons are considered to be exciteddirectly in order to produce a wave emitted from a surface which has thedesired frequency band filter behavior.

FIGS. 9A and 9B show the conversion efficiency gains using lithographicfeature design and fabrication. Such IR sources illustrate a spectralnarrowing of 0.5, which gives them almost a factor of four efficiencyadvantage over an ideal blackbody. By bringing emission width to 0.1(FIG. 9A), the width achieved to date for filter transmission width,another factor of 4–5× in conversion efficiency is obtained (FIG. 9B).

The size and shape of the repetitive surface feature control the detailsof the emission spectrum: the peak frequency in the transmission band;the width of the transmission band; the location of the minima whichdefine the band; the steepness of the intensity falloff.

A two-dimensional array with 90° rotation symmetry serves to minimizepolarization dependence of the effect, and variation of the details ofthe repetitive feature (replace a square unit mask by a plus-sign shape,e.g.) enhances desired spectral characteristics. The resonant frequencyof the transmitted wave scales linearly with the length of the unit cellin the grid, as expected by analogy with grating studies. Other detailsdepend sensitively upon feature shape (as mentioned previously), thewidth of lines which create the feature, and feature-to-featurevariations. Other features of a textured surface—height of surfacefeatures, wall profile shape have an effect, as well.

Existing theoretical studies on transmission properties use models andanalogs for prediction. Scalar diffraction theory has been used, as wellas a transmission line analog: both with reasonable success. Thedrawback of such models is that their applicability to a particularsituation can only be certified by experimental tests. A prioridetermination of a region of applicability of particular models is notassured.

In accordance with the invention, transmission line analogs comprise themodels for prediction of emission from grids and periodic masks. Inthese models, each element (unit cell) of the grid is modeled as asimple circuit consisting of a resistance, capacitance and inductancewhose values are adjusted so that circuit current matches output fieldstrength of the unit cell. A grid then consists of a large number ofidentical circuits coupled together with capacitors (for capacitivegrids, consisting of conducting elements out of contact with oneanother) or inductors (for inductive grids, consisting of continuousconductors as in a wire mesh.) The collection of connected circuitsimplies an output current which are solved by techniques common intransmission line analysis. Analyses such as these are used incalculating transmission properties of grids and masks.

A first principle approach, using Maxwell's equations, is used toestablish the validity of this model, and to determine the appropriateR, C, and L values for a given unit element size and shape. The couplingvalues are similarly found. Once the model has been shown valid for thefrequency range of interest, the model is used to simplify analysis ofthe complete grid. When variation in the unit cell parameters is to beinvestigated, the analysis becomes more complicated, and the results arecast into a matrix inversion problem where standard computer techniquesare employed.

Maxwell's Equations are used as a basis to predict the surface emission,and polarization independence, and then to calculate the emissionspectrum of the unit surface cell. Assuming 90 degree rotation symmetryand an infinite grid size, this result is expanded to represent emissionfrom a large surface. Then, small square grids (10×10 features, forexample) with random feature-to-feature variation are modeled todetermine a representation of the effects of fabrication artifacts andvariations.

Integrated Circuit Sensor

Pitch/Catch Source and Detector on the Same Die

The invention further includes a turbulence-based infrared hydrocarbonleak detector which is radically simpler than conventional infraredabsorption instruments. Two aspects make this possible. First, a simplestructured IR instrument is configured by building a tuned infraredsource and conventional infrared detector into a single package. Second,digital signal processing techniques are used to overcome the DC and lowfrequency drift which characterizes most nondispersive infraredmeasurements. The sensor does not provide an absolute concentrationmeasurement for hydrocarbons but it is particularly sensitive to thehigh frequency “noise” caused by the turbulence accompanying changes inlocal gas concentration. This provides a robust leak detectioncapability for gases which are not normally present in the samplingenvironment. Combined with a simple reflector plate to define the gassampling region, this sensor provides a rugged, reliable, field- andflight-worthy hydrocarbon leak detection capability.

FIG. 10 shows a novel, low-cost infrared hydrocarbon leak sensor usingan integrated source and detector in an open path atmospheric gasmeasurement. As a leak detector, the sensor discards low frequency“signal” in favor of the high frequency “noise” associated with changesin local gas concentration. In one form, the entire sensor engine mountson a TO-8 transistor header.

Hot Bolometer Sensor (With and Without Separate Filter)

An aspect of the invention can be used to form an infrared gasmonitoring component to build an integrated on-board exhaust NOx meter.Silicon micromachining technology is used to build a sensor which isradically simpler than conventional infrared absorption instruments.Infrared absorption instruments traditionally contain a source ofinfrared radiation, a means of spectral selection for the gas understudy, an absorption cell with associated gas sample handling and/orconditioning, any necessary optics, a sensitive infrared detector, andassociated signal processing electronics. The invention simplifies andreduces the cost of an IR instrument by integrating the function of theinfrared source and infrared detector into a single self-supportingthin-film bolometer element. This element is packaged with inexpensivemolded plastic optics and a conventional spectral filter to make atransistor-size “sensor engine.” Combined with a simple reflector plateto define the gas sampling region, this sensor engine provides acomplete gas sensor instrument which is extremely inexpensive and whichwill approach the sensitivity of conventional infrared absorptioninstruments.

FIG. 11A shows a novel, low-cost infrared gas sensor engine 100 using aheated bolometer element as both source and detector in an open pathatmospheric gas measurement. The bolometer element reaches radiativeequilibrium with its surroundings at a slightly lower temperature if gasabsorption frustrates light re-imaging on the element. The compoundparabolic concentrator defines a relatively narrow illumination cone andthe passive reflector is designed to provide a pupil-image of thespectral filter onto itself. The entire sensor engine can be mounted ona TO-8 transistor header. FIG. 11B shows a detailed view of the sensorengine 100.

Achieving tight spectral control of the infrared emission is importantin making the heated bolometer element work well. The device isparticularly effective if the amount of radiation absorbed by gasmolecules under study is made measurably large in terms of the overallthermal budget of the bolometer surface. Preferably, a tuned cavity bandemitter is constructed with spectral resolution (dl/1) around 0.1,roughly the performance achieved to date with micromesh reflectivefilters. This raises the conversion efficiency to nearly 15% for the NOxproblem. This level of surface topology (and therefore spectral)control, is achieved through micro-electro-mechanical systems (MEMS)technologies.

The embodiments of the sensor engine shown in FIGS. 11A and 11B includea single bolometer which both emits and detects radiation. Thoseconfigurations permit construction of devices, such as gas detectors,which are compact and employ a minimum number of component parts. Insome instances, however, it is useful to construct such devices withseparate radiations sources and sensors. Exemplary devices usingseparate radiation sources and detectors are shown in FIGS. 11C–11I.

In FIG. 11C, radiation source 102 and radiation detector 104 are mountedon a silicon die 106, facing a reflector 108 disposed across a testregion 110. The reflector 108 is positioned to effect an optical pathbetween the source 102 and detector 104. The test region 110 isconstructed so that an applied test gas flow passes between the source102/detector 104 pair and the reflector 108, intercepting the opticalpath. The silicon die 106 includes integrated processing circuitry 112coupled between the detector 104 and a data output port 116. Anelectrical control 118 is coupled to source 102 for driving source 102to emit radiation, preferably infrared radiation with a spectral rangeincluding an absorption line of the gas to be detected.

FIG. 11D shows a configuration similar to that in FIG. 11C, but wherethe source 102 and detector 104 are housed in a closed chamber C, andthe reflector (mirror) 108 is external to the chamber C, with theoptical path between the source 102/detector 104 and reflector 108passing through an optically transmissive window W. With thisconfiguration, the gas under test is isolated from the source anddetector and any other instrumentation.

FIG. 11E shows an embodiments similar to that in FIG. 11C, but where thesource 102 and detector 104 are mounted within an enclosure E disposedon a single can 120. The enclosure E establishes a test gas flow paththerethrough, which intercepts the optical path between source102/detector 104 and the reflector 108.

FIG. 11F shows an embodiment wherein a source 102 is disposed oppositeto a detector 104, wherein a collimating lens 122 establishes acollimated radiation beam through a test gas flow to a lens 124. Lense124 focuses the resultant beam onto a detector 104.

FIG. 11G shows a source 102 which is activated by an optical beam Bdirected from a high energy laser L (by way of lens 132, reflector 134and lens 136). The source 102 is back illuminated, so that emittedradiation propagates from an emission surface toward a reflector 108A,which in turn directs that radiation across a test gas flow to areflector 108B. Reflector 108B focuses the radiation incident thereon toa detector 104 which is coupled to a data output port 116.

FIG. 11H shows source 102 and detector 104 on opposite sides of asupport, each facing a respective one of reflectors 108A and 108B.Source 102 is electrically driven to emit radiation which propagates toreflector 108A. Reflector 108A directs the radiation incident thereonacross the test gas flow to reflector 108B. Reflector 108B focuses theradiation incident thereon to the detector 104.

FIG. 11I shows source 102 and detector 104 mounted on the same substrateS, and opposite to a reflector 108. A lens 142 directs radiation fromsource 102 to reflector 108. Reflector 108 directs radiation incidentthereon to a lens 144, which in turn focuses that radiation ontodetector 104. An optically transmissive test gas cell 148 (containing agas to be identified) is disposed between the lenses 142 and 144 and thereflector 108. Detector 104 provides via data out port 116, a signal toa reference data comparator 150, which compares a received signal (fromdetector 104) to reference information, for example to permitidentification of one or more components in the gas in test gas cell148.

Wheatstone Bridge Drive Circuit

An individual emitter die is packaged, together with individual infrareddetector pixel elements and thin film interference filter windows inTO-8 transistor cans using standard process equipment.

Drive and readout schemes using a microprocessor controlled,temperature-stabilized driver are used to determine resistance fromdrive current and drive voltage readings. That information shows thatincidental resistances (temperature coefficients in leads and packagesand shunt resistors, for instance) do not overwhelm the small resistancechanges used as a measurement parameter.

A straightforward analog control circuit, the Wheatstone bridge is usedto perform that function. It is very simple, very accurate, quiteinsensitive to power supply variations and relatively insensitive totemperature. The circuit is “resistor” programmable but depends forstability on matching the ratio of resistors. In one form, an adjacent“blind” pixel—an identical bolometer element filtered at some differentwaveband—is used as the resistor in the other leg of the bridge,allowing compensation for instrument and component temperatures andproviding only a difference signal related to infrared absorption in thegas.

An optics test bed is used to evaluate different configurations andperform measurements of the device under benchtop conditions. Forelevated ambient (automotive) temperature operation, the device isoperated as instrumented tube furnaces and to calibrate the infraredreadings against a conventional gas analyzer.

The Wheatstone bridge provides a simple computer interface and since itis implemented with relatively robust analog parts is not susceptible toradiation damage at high altitudes or in space.

For the Wheatstone bridge shown in FIG. 12, bridge is balanced whenR1/R2=R3/R4, and to first order, temperature coefficients of R1 and R2can be neglected if resistors are matched. The temperature coefficientof R3 is important but should have negligible effect across delta Tcaused by the gas absorption. Preferably, the bridge is carefullybalanced for the designed operating temperature. The estimated errorsfrom an analog readout of this circuit come from the amplifier inputoffset and input bias currents which introduce offset voltage or errorterm. FIG. 13 shows a test configuration.

Turbulence Detection and Signal Processing

In connection with industrial gas sensors, we have found thatsource-detector-electronics combinations can readily be stabilized toprovide signal to noise performance of 1000:1 or better under conditionsof stable temperature and steady state gas flow. Typical conditions arepulse frequencies of 1–10 Hz, well matched to the speed of availablethermal detectors. However, high frequency transients associated withchanges in gas flow provide a significant disruption. Ordinarily, infact, detector preamplifier circuits are severely filtered to suppressthese high frequency transients.

Most detectors have poor low frequency characteristics and are subjectto 1/f or low frequency noise and drift. This is also true for most DCamplifiers where precision components are required for good lowfrequency response. This noise is avoided by selectively filtering andamplifying only those high frequency components present in a turbulentgas stream. In order to separate these high frequency components, anarrow, band pass filter is used; these filters are preferablyconstructed using low cost integrated circuits.

Emitter elements (narrow spectral band, high cold resistance) are usedwith filtered pyroelectric detectors. A/D converters are used to collectand analyze the data and optimize the signal processing sequence.

Although the high frequency components are difficult to see in the powerspectral density plots, the thermopile detectors used have a poor highfrequency response. Lead selenide and pyroelectric detectors have a muchbetter high frequency response than the thermopile detector.

FIG. 14A shows a typical nondispersive infrared gas sensor test bedincluding infrared radiator and detector, suitable filters, and a smallenvironmental chamber for performing thermal cycle and gas flow tests.FIG. 14B shows a typical signal processing stream for data reduction.

Automated data collection analysis software, or off-line data analysissoftware is used for signal processing, rapid prototyping of datacollection and analysis systems. Real-time power spectral density plotsand other signal processing techniques identify and isolate thosefrequency characteristics, that will enable this simple, yet effectivegas detection approach.

FIG. 15 shows measured data from a pulsed-source and thermopile detectorcombination, illustrating the high (20–40 Hz) ripples associated withchanges in gas flow. Long considered a problem for IR gas detectors,this characteristic is used to exploit this “noise” as the primary“signal” for a leak detector.

Multi-Pixel Sources for Scene Generators and FPA Health CheckMulti-Pixel Calibrator for Hyperspectral Imagers

A tiny, self-referencing band emitter is used for absolute radiometriccalibration of infrared spectral channels. This simple, rugged devicecan be incorporated directly into a radiometric instrument, or into afield test kit. Microelectromechanical (MEMS) fabrication techniques areused to produce a photonic band gap infrared emitter surface with anarrow emission band tuned to the infrared spectral channel under test.A discrete single-channel (less than 1 mm active area) radiometriccalibration element is used. This configuration is stable, repeatable,and has high power conversion efficiency and out-of-channel rejection.The configuration can provide a transistor-sized multi-pixel,multi-channel hyperspectral calibration tool.

A simple built-in radiometric and wavelength calibration device is usedfor hyperspectral imaging systems. A silicon bridge emitter element hasa photonic band gap (PBG) surface tuned to emit over a narrow infraredwaveband. With very low thermal mass and high temperature coefficient ofresistivity (TCR), this element acts as its own thermistor forself-referencing, electronic feedback control.

FIG. 16 shows a simple, built-in calibrator for hyperspectral imagingsystems, using silicon micro bridges with a photonic band gap surfacestructure to achieve narrow band emission and absolute feedbacktemperature control in the infrared.

Hyperspectral Imaging

Passive infrared spectra contain critical information about chemicalmake-up while broad-band infrared images provide information on thelocation, presence, and temperature of objects under observation.Hyperspectral imaging, the ability to gather and process infraredspectral information on each pixel of an image, can ultimately providetwo dimensional composition maps of the scene under study and this datais critical for ecosystem inventory and status monitoring. These studiesroutinely require data in the near- and mid- infrared spectral bands,over large areas, at a reasonably high resolution suited to ecologicalstudies. Data goals include complementary surface moisture and surface(vegetation) temperature data taken in registration with the shorterwavelength data (allowing precise and straightforward correlation withLWIR data) for plant coverage, deforestation, and plant healthmonitoring studies. Variations in surface albedo within from frame toframe and even within a single frame make it difficult to infer accuratetemperature measurements from bandpass flux at a single wavelength. Thismay be particularly acute for deforestation studies where sudden loss ofvegetation coverage would be expected to produce surface humiditychanges with simultaneous shifts in surface temperature and surfacealbedo.

A traditional solution is to measure infrared bandpass flux in adjacentspectral bands to allow extraction of accurate color temperatures.Hyperspectral imaging can provide surface temperature and humiditymapping, at a resolution compatible with the shorter wavelength systems,over inaccessible regions where this data is not available from groundmonitoring stations or other sources.

Current hyperspectral imaging systems such as the NASA/JPL AirborneVisible Infrared Imaging Spectrometer (AVIRIS) and Kestrel's FourierTransform Hyperspectral Imager (FTHSI) are bulky and expensive whichrenders widespread use of the technology impractical, especially forcommercial applications. Rapidly evolving uncooled hyperspectral imagingsystems, based on recent advances in infrared focal plane arraytechnology, can provide simultaneous mid-wave (MWIR) and long-wave(LWIR) data for coastal ecology studies. These devices, which mustoperate under field and flight conditions over large temperature andhumidity ranges, require periodic radiometric and spectral calibrationto assure long term accuracy.

Radiometric Calibration

Hyperspectral image measurements are difficult to make in the fieldsince the instruments must operate unattended (or with minimalattention) in hostile environments, over extreme field temperature andhumidity ranges. Much of the important irradiance information is in the1–3 mm spectral band where low signal and drift in infrared sources andinfrared detectors make stability and calibration difficult. In fact,the standard NIST FAL, T-10 reference bulbs generally used forlaboratory calibration have very low signal in this waveband. But theatmospheric research community has made steady progress in the accuracyand reliability of these measurements and there is now a broad consensuson absolute standards for bolometric radiometry established throughbi-annual international inter-comparisons between instruments used byvarious groups and a gold triple point standard. In general terms, thestate of the art for absolute accuracy in these measurements hasprogressed to around 0.3% (3s).

The most effective strategy for maximizing instrument sensitivity iscalibration to identify and compensate for gain and offset variations.This is done with conventional blackbody sources which depend upon largesize and thermal mass to achieve uniform temperature. These sources arenot easily incorporated directly into the instrument. As a consequence,calibration is infrequent and, particularly for instruments stationed atinaccessible locations, very infrequent.

Accordingly, to one aspect of the invention, a miniature,electrically-modulated multispectral line source weighing a few gramsand occupying a fraction of a cubic centimeter provides the solution tothis problem. The heart of this source is an array of photonic band gapnarrow band emitter pixels. Each pixel radiates only within itsdesignated spectral band and achieves temperature stability by heatingand cooling on a time scale much faster than thermal diffusion. Its lowthermal mass, near-perfect in-band emissivity, and high temperaturecoefficient of resistivity cause it to precisely follow its drivecurrent, permitting instantaneous feedback and precise temperaturecontrol. The source is readily packaged into radiometric instrumentswhere its low power demand permits remote, battery-powered operationunder computer control.

The stability of conventional, cavity-style blackbody sources depends ongood thermal conductivity (to minimize spatial temperature gradients)and a large thermal inertia (to minimize temporal fluctuations). Thisdetermines their weight and power consumption, both of which aretypically large. The IR source of this aspect of the invention, becauseit is based upon a few-micron-thick textured emitter element, does relyon its thermal mass for stability, but instead depends upon a stableelectronic drive supply. This is an important advantage for the sourceof the invention because super-stable, precisely controlled electronicsare much easier to engineer than a highly stable thermomechanical systemand, when built, respond orders of magnitude more quickly to programmedchanges, weigh less, occupy less space, and are less expensive thanmassive radiators.

Conventional blackbody sources require long lead times for warm-up andstabilization and, during these times, they place a parasitic heat loadon components in the optical train. For components at cryogenictemperature, the source-induced heat load is especially burdensomebecause it is continuous, and heats the optical train even when not inuse as a reference or for calibration. Miniature IR sources of theinvention, on the other hand, rely upon electrically heated filaments ofsuch low thermal mass that their temperature at all times correlatesprecisely with the current flowing through them. They can be heated (andcooled) in milliseconds, delivering a current-following temperatureprofile exactly matching the drive pulse. Because they rely onelectrical pulse shaping and current control rather than thermal massfor stability, these miniature sources have several importantadvantages. First, they need only be powered for a short time whenrequired, thus eliminating parasitic heating. Second, their low thermalmass and high radiant output allow them to achieve a very hightemperature-slew-rate with virtually no thermal hysteresis. This meansthat at any instant, filament output is directly related to electricaldrive power. Effectively, this converts the thermal stability probleminto a matter of assuring electrical stability.

Portable MTF (Modulation Transfer Function) Tester

According to another aspect of the invention, an infrared resolutiontester which is a thermal version of the familiar resolution testpatterns for visible band optical systems. Based on the use of miniaturetextured metal radiators for flat-field correction of flight systems,the invention is based on lithographic techniques to build apostage-stamp sized device with alternating bands of polished(emissivity, ε≅0.01) and textured (ε≅0.99) metal to form the familiarUSAF 1951 optical resolution test pattern. The integrated resolutiontester based on this device is light-weight, low power, and respondsrapidly to turn on/turn off commands. This device is mounted on atemperature controlled stage and demonstrates performance withcollimating optics. This device is a flightworthy resolution tester forfield and flight monitoring of infrared imaging systems.

For any imaging system, one of the most important figures of merit isresolution. This is typically expressed as a modulation transferfunction (MTF), or contrast as a function of spatial frequency. Forvisible-band systems, MTF has historically been measured with standardresolution test charts; because no such devices are available in theinfrared, designers typically rely on slits (often home-made) in a metalfoil mounted in front of a large cavity-style blackbody to make themeasurement. However, recent advances in ion-beam microtexturing nowenable another aspect of the invention, namely the manufacture ofadjacent regions of very high and very low emissivity in a complexpattern of very closely controlled characteristics.

FIG. 17A shows a resolution test pattern and FIG. 17B shows a resolutiontester. By using ion beam lithography techniques, alternating bands ofvery high and very low emissivity are established on a lightweight, highconductivity metal foil surface. Using a tiny thermoelectric hot/coldstage to uniformly vary the temperature of this test pattern permitssimultaneous radiometric and resolution tests.

Modern high-performance IR systems have narrow fields of view,consequently long focal lengths and very high resolution. Thiscomplicates the problem of designing a built-in resolution tester, sincecalibration objects must appear to be very far away. In a conventionaltest arrangement, a collimator is used to project the image of theblackbody source with a slit pattern onto the aperture of the systemunder test. In this arrangement, the collimator acts as one lens and thefore-optics as another; the magnification of this two lens system is thefocal length ratio of the two lenses. This means that, to project verysmall test patterns onto the focal plane array under test, either thetest target itself must be very small or the collimator must have a muchlonger focal length than the fore-optics of the system under test. Atest object of the invention has extremely fine lines and spaces, as thekey to calibrating high resolution systems with a device of modest size.In one form, the invention includes a three-element, germanium f/1 lenssystem adapted from the “IR microcam,” to build a hand-held externaltest device.

FIGS. 18A and 18B shows the IR scam lens as a 3-element germanium f/1system. It has a 25 mm aperture. It provides night vision capability forhand-launched unmanned aircraft (HL-UAV). This optical train is thebasis of a hand-held resolution tester.

FPA Health Check Source

Like most infrared seekers, gain and offset in the AIT seeker depend onthe local temperature (and pixel-to-pixel temperature variations) on thefocal plane array. One way to compensate for gain and offset variationsis to use a flat-field IR reference source to provide uniformillumination on all pixels before (or during) flight. Such a sourcewould also be extremely useful for an FPA health check on a Dem-Valflight. The metal texturing technology developed by the M&S programmakes it possible to build extremely compact, lightweight infraredreference sources. Another aspect of the invention is an MWIR referencesource for the BMDO STRV-2 mission. The STRV-2 source is very small (6mm diameter×3 mm thick) since it was designed to go on the filter wheelvery close to the FPA.

Changes in the STRV-2 orbital and mission profiles meant that itsoriginal cryocooler was replaced with a more robust, but vibrationallynoisier, cooler. Although this solved some thermal problems for the MWIRtelescope, it meant that the telescope had to take its images with thecooler switched off. The new orbit, inclined nearly 65 degrees, willallow the telescope to see the sun for nearly half of every orbit sothat the telescope must be designed to operate accurately even when itis not in thermal equilibrium. This will require on-station calibration.But, because weight and space are at a premium within the MWIR payload,a conventional blackbody source was not practical.

FIG. 19A shows a prototype reference source for STRV-2, mounted in anexisting filter wheel slot for minimum design and schedule impact. Thissource provides flat-fielding to correct pixel-to-pixel offsetvariations. FIG. 19B shows exemplary sources adjacent to a metric.

This simple modular device provides a flat field warm-body referencewith no parasitic heating of the optical train and it will allowcorrection of pixel-to-pixel offset variations on the focal plane.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein

1. A narrow band incoherent radiation emitter detector comprising: aplanar filamental emission/detection element characterized by apredetermined spectral range of emitted/detected radiation and aemission/detection width of dl/1 less than about 0.1, where 1 is thewavelength of said radiation, wherein said emission/detection width issubstantially determined by surface features of said emission/detectionelement.
 2. An emitter detector to claim 1 wherein said spectral rangeincludes relatively long wavelengths and excludes relatively shortwavelengths.
 3. An emitter/detector according to claim 2 wherein saidemission/detection width is substantially determined by surface featuresof said emission/detection element.
 4. An emitter/detector according toclaim 1 wherein said spectral range is near an infrared absorption lineof a predetermined material.
 5. An emitter/detector according to claim 4wherein said emission/detection width is substantially determined bysurface features of said emission/detection element.
 6. Anemitter/detector according to claim 1 wherein said spectral rangeexcludes relatively long wavelengths and relatively short wavelengthsand includes a range of intermediate wavelengths therebetween.
 7. Anemitter/detector according to claim 6 wherein said range of intermediatewavelengths includes an infrared absorption line of a predeterminedmaterial.
 8. An emitter/detector according to claim 6, furthercomprising a thermal detector for photons characterized by a wavelengthwithin said intermediate range.
 9. An emitter/detector according toclaim 6 further comprising a thermal detector for detecting Infraredenergy characterized by a wavelength in said intermediate range.
 10. Anemitter/detector according to claim 6 wherein said emission/detectionelement is a suspended filament made of a metal foil.
 11. Anemitter/detector according to claim 6 wherein said emission/detectionelement is suspended filament made of a back-etched semiconductor. 12.An emitter/detector according to claim 6 wherein said emission/detectionelement is a resistive element having an emission surface to controlsaid spectral range.
 13. A gas detector comprising: A. a planarfilamental emission/detection element characterized by a predeterminedspectral range of emitted/detected radiation and a emission/detectionwidth dl/1 less than about 0.1, where 1 is the wavelength of saidradiation, wherein said emission/detection width is substantiallydetermined by surface features of said emission/detection element, saidemission/detection element having an input/output axis, and B. a firstreflector disposed along said input/output axis and opposite to saidemission/detection element, whereby an optical path is defined from saidemission/detection element to said first reflector to and back to saidemission/detection element, wherein said optical path between saidemission/detection element and said first reflector passes through a gastest region.
 14. A gas detector according to claim 13, furthercomprising: C. a driver for driving said emission/detection element toemit radiation propagating along said optical path toward said firstreflector.
 15. A gas detector according to claim 12 further comprising:D. a processor responsive to said emission/detection element forgenerating an output signal representative of radiation incidentthereon.
 16. A gas detector according to claim 13 wherein said spectralrange includes a wavelength corresponding to an absorption line of apredetermined gas.
 17. A gas detector according to claim 13 furthercomprising: a second reflector extending from points near saidemission/detection element along said input/output axis, wherein saidsecond reflector is disposed along said optical path, whereby saidoptical path extends from said emission/detection element to said secondreflector to said first reflector to said second reflector to saidemission/detection element, and wherein said optical path between saidsecond reflector and said first reflector passes through said gas testregion.
 18. A gas detector according to claim 17, further comprising: C.a driver for driving said emission/detection element to emit radiationpropagating along said optical path toward said first reflector.
 19. Agas detector according to claim 18 further comprising: D. a processorresponsive to said emission/detection element for generating an outputsignal representative of radiation incident thereon.
 20. A gas detectoraccording to claim 17 wherein said second reflector is a beam-formingreflector and said second reflector is substantially planar.
 21. A gasdetector comprising: A. a planar filamental emission elementcharacterized by a predetermined spectral range of emitted radiation andan emission width dl/1 less than about 0.1, where 1 is the wavelength ofsaid emission element having an output axis, wherein said emission widthis substantially determined by surface features of saidemission/detection element, B. a first reflector disposed along saidoutput axis, and C. a planar filamental detection element characterizedby a predetermined spectral range of detected radiation and anemission/detection width dl/1 less than about 0.1, where 1 is thewavelength of said detection element having an input axis, whereby anoptical path is defined from said emission element to said firstreflector and to said first detection element, wherein said optical pathbetween said emission element and said first reflector, or between saidfirst reflector and said detection element or both, passes through a gastest region.
 22. A gas detector according to claim 21 furthercomprising: a second reflector disposed along said optical path betweensaid first reflector and said detection element whereby said opticalpath extends from said emission element to said first reflector to saidreflector to said detection element, and wherein said optical pathbetween said first reflector and said second reflector passes throughsaid gas test region.
 23. A gas detector according to claim 21, furthercomprising: C. a driver for driving said emission element to emitradiation propagating along said optical path toward said firstreflector.
 24. A gas detector according to claim 21 further comprising:D. a processor responsive to said detection element for generating anoutput signal representative of radiation incident thereon.
 25. A gasdetector according to claim 21 wherein said spectral range includes awavelength corresponding to an absorption line of a predetermined gas.26. A multi-wavelength radiation emitter/detector array comprising: anarray of planar emission/detection elements, each element beingcharacterized by a predetermined spectral range of emitted/detectedradiation and an emission/detection width dl/1 less than about 0.1, when1 is the wavelength of said radiation, wherein each emission/detectionwidth is substantially determined by surface features of each respectiveemission/detection element.
 27. An array according to claim 26 whereinsaid array is adopted to emit/detect information representative of aplanar image.